The present application claims priority under 35 U.S.C. 119 of Japanese laid-open patent application serial no. 11-253033, filed Sep. 9, 1999, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to co-operative control systems for vehicles comprising a combination of a force distribution device, which distributes the driving force or braking force between the right and left wheels or between the front and rear wheels, and an electric power steering device that applies a steering assist torque to a steering system.
2. Description of the Relevant Art
There is a known technique for enhancing turning performance of a vehicle, where the ratio by which the engine driving force is distributed between the right and left driven wheels is made variable, and the driving force distributed to the outer turning wheel is increased while the driving force distributed to the inner turning wheel is decreased so as to generate a yaw moment in the turning direction. In a vehicle having such a driving force distribution device, there is a disadvantage in that when the driving forces distributed to the right and left driven wheels are varied, an undesirable steering force is produced in the right and left driven wheels that also serve as steered wheels (the torque steer phenomenon). Therefore, the present assignee has already made a proposal in which the torque steer phenomenon is lessened by utilizing an electric power steering device, which is provided in a vehicle, and generating a steering assist torque in the electric power steering device so as to counteract the above-mentioned undesirable steering force (ref. Japanese Patent Application No. 9-302155).
In a vehicle carrying out the above-mentioned co-operative control, when the electric power steering device malfunctions and a steering assist torque cannot be generated, the torque steer phenomenon caused by the operation of the driving force distribution device cannot be counteracted and there is a possibility that the driver might experience a disagreeable sensation.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an objective of the present invention to prevent a disagreeable sensation caused by the aforementioned torque steer phenomenon from being experienced by the driver in the event that the electric power steering device or the control means therefor malfunctions in a vehicle co-operative control system for reducing the torque steer phenomenon by co-operatively controlling the force distribution device and the electric power steering device.
In order to achieve the above-mentioned objective, in accordance with a first aspect of the invention, a co-operative control system for a vehicle is proposed, the system includes a force distribution device for distributing the driving force or braking force between the right and left wheels or between the front and rear wheels. In the practice of the invention, a first control means is provided for controlling the operation of the force distribution device. Also in the practice of the invention, the control system also affects an electric power steering device, having a motor for applying a steering assist torque to the vehicle steering system, and a second control means is provided for calculating a motor control signal for driving the motor. The motor control signal is based on at least the steering torque detected by a steering torque detecting means. The first control means can calculate a correction signal for correcting said motor control signal based on the distributed force generated by the force distribution device. The second control means drives the motor based on the corrected motor control signal obtained by correcting the motor control signal with the correction signal. The first control means stops the operation of the force distribution device when the electric power steering device or the second control means malfunctions.
In accordance with the above-mentioned arrangement, when the electric power steering device or the second control means malfunctions and, as a result, the torque steer phenomenon accompanying the operation of the force distribution device cannot be lessened, the first control means stops the operation of the force distribution device, so preventing the aforementioned torque steer phenomenon from occurring. It is thus possible to prevent the driver from experiencing a disagreeable sensation.
In accordance with a first refinement of the basic invention, a co-operative control system for a vehicle is proposed wherein when the malfunction occurs during the operation of the force distribution device, the first control means gradually decreases the absolute value of the difference between the distributed forces of the force distribution device to the right and left wheels towards zero.
In accordance with the above-mentioned arrangement, when the aforementioned malfunction occurs during the operation of the force distribution device, since the absolute value of the difference between the distributed forces of the force distribution device to the right and left wheels gradually decreases towards zero, a rapid change in the steering reaction, which could be experienced by the driver from the steering wheel can be prevented.
In accordance with a second refinement of the invention, a co-operative control system for a vehicle is proposed wherein when the malfunction occurs while the force distribution device is generating a distributed force which is larger than a reference force, at which the rack shaft force of the electric power steering device becomes zero without a correction by the correction signal, the first control means immediately decreases the distributed force of the force distribution device to a level less than said reference force and then gradually decreases the absolute value of the difference between the distribution forces to the right and left wheels of the distribution device towards zero.
If the aforementioned malfunction occurs when the force distribution system generates a distributed force which is larger than the reference distributed force, torque steer is caused which turns the vehicle sharply towards the inside in the turning direction, but in accordance with the invention, since the distributed force is immediately decreased to a level smaller than the aforementioned reference distributed force when a malfunction occurs, it is possible to prevent the occurrence of the torque steer phenomenon which turns the vehicle sharply towards the inside in the turning direction. Moreover, since the absolute value of the difference between the distributed forces of the distribution device to the right and left wheels is subsequently decreased gradually towards zero, the steering reaction is prevented from changing rapidly.